This invention relates to apparatus and methods for controlled delivery of cryogenic liquid, such as liquid nitrogen.
In various applications, it is important to deliver a metered amount of cryogenic liquid. For example, thin-wall containers, such as plastic, aluminum or steel beverage cans, can be used for non-carbonated beverages by adding a metered amount of inert cryogenic liquid immediately before capping the can. When vaporized, the inert cryogen increases internal can pressure which strengthens it, helping the can resist collapse, for example, when stacked for storage or for transport.
Controlled delivery is very important in such applications. Too little cryogen will not provide adequate pressure (strength), and the can may fail to withstand forces encountered in stacking and shipping. Too much nitrogen can create excessive internal can pressure, deforming the can and possibly exploding it.
The ability to meter cryogenic liquids is complicated by ambient water vapor which condenses and freezes on surfaces of the delivery apparatus, clogging it and contaminating the containers by dripping into them. In the environment of a production line, there may be extreme temperature and humidity conditions which exacerbate these problems. For example, an automated beverage can assembly line may involve injection of hot, recently pasteurized beverage into the can at a station adjacent to the apparatus for delivering liquid nitrogen. Large amounts of frost can build up on the delivery apparatus.
Another obstacle to metering the flow of liquid cryogen is the tendency of the cryogen to vaporize in delivery conduits, particularly when undergoing a pressure drop, e.g. at an outlet where liquid cryogen is supplied under pressure. Because of the large difference in liquid and vapor density, even a small amount of vaporization dramatically alters the volume ratio of liquid/vapor, thereby altering the rate of cryogen delivered over time.
The ability to meter cryogenic liquids is further complicated by splashing of the cryogen as the can moves along the assembly line rapidly, through sharp turns.
When the cryogen used is liquid nitrogen, which boils slightly below the boiling point of oxygen, another problem is oxygen condensation at the site of the cryogen, which can enrich the oxygen present in packaged food, having a detrimental effect on the food. The further the open container travels with liquid cryogen in it, the more serious this problem becomes, and cryogen delivery apparatus often is too bulky to be placed immediately adjacent the site where the cap is installed.